A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane 102 which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window membrane openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns in a human cochlea. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The axial center of the cochlea 104 is called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are sensed by the acoustic nerve 113 and sent to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea. To improve impaired hearing, hearing prostheses have been developed. For example, when the impairment is related to operation of the middle ear, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound to the tympanic membrane. Or when the hearing impairment is associated with the cochlea, a cochlear implant with an implanted electrode carrier can electrically stimulate adjacent auditory neural tissue with small currents.
In some patients with some residual hearing in the lower acoustic frequencies, a conventional hearing aid and a cochlear implant can be combined together in a hybrid Electric Acoustic Stimulation (EAS) system. The hearing aid acoustically amplifies lower acoustic frequencies perceived by human ear, while the cochlear implant electrically stimulates the middle and high frequencies. See von Ilberg et al, Electric-Acoustic Stimulation of the Auditory System, ORL 61:334-340; Skarzynski et al, Preservation of Low Frequency Hearing in Partial Deafness Cochlear Implantation (PDCI) Using the Round Window Surgical Approach, Acta OtoLaryngol 2007;127:41-48; Gantz & Turner, Combining Acoustic and Electrical Speech Processing: Iowa/Nucleus Hybrid Implant, Acta Otolaryngo12004;124:344-347; Gstöttner et al., Hearing Preservation in Cochlear Implantation for Electric Acoustic Stimulation, Acta Otolaryngol 2004; 124:348-352; all incorporated herein by reference.
FIG. 1 also shows some components of a typical EAS system which includes an external microphone that provides an acoustic signal input to an external signal processor 111 where two different signal processing paths are developed. An upper acoustic frequency range communications signal containing middle and high frequency range acoustic is converted into a digital data format, such as a sequence of data frames, for transmission via a transmitter coil 107 over a corresponding implanted receiver coil 106 into the electric implant 108. Besides receiving the processed acoustic information, the electric implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces an electric stimulation pattern (based on the extracted acoustic information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple electrode contacts on its outer surface that provide selective electric stimulation of the cochlea 104. The external signal processor 111 also creates a lower acoustic frequency range communications signal to a conventional hearing aid 105 in the ear canal which acoustically stimulates the tympanic membrane 102, and in turn the middle ear 103 and cochlea 104.
FIG. 2 shows a graph illustrating how the acoustic frequency range is fully covered by an EAS system with the lower range of residual hearing covered by natural hearing and/or a conventional acoustic hearing aid in the ear canal while the upper acoustic frequency range where the patient shows hearing loss is covered by electrical stimulation from the cochlear implant.